•Fig. 16.15 Effects of Excitation at Various Times After the Initiation of an Action Potential in a Slow-Response Fiber. Inthis fiber,excitationverylateinphase3(orearlyinphase4)inducesa small,nonpropagated(local)response(wavea).Laterinphase4,a •Fig. 16.14 Changesinactionpotentialamplitudeandupstrokeslopeasactionpotentialsareinitiatedatdifferentstagesoftherelativerefractoryperiodoftheprecedingexcitation.(RedrawnfromRosenMR,etal.Am Heart J. 1974;88:380.)
In slow-response fibers, the relative refractory period frequently extends well beyond phase 3 (see
Fig. 16.1B ). Even after the cell has completely repolarized, it may be difficult to evoke a propagated response for some time. This characteristic of slow-response fibers is called postrepolarization refractoriness.
propagatedresponse(waveb)canbeelicited,butitsamplitudeissmallandtheupstrokeisnotverysteep;thisresponseisconductedveryslowly.Stilllaterinphase4,fullexcitabilityisregained,andtheresponse(wavec)displaysnormalcharacteristics.(ModifiedfromSingerDH,etal.Prog Cardiovasc Dis. 1981;24:97.)
Fig.16.32 ),thetimingoftheseearlybeatsmaydeterminetheirclinicalconsequence.Iftheyoccurlateintherelativerefractoryperiodoftheprecedingdepolarizationorafterfullrepolarization,theprematuredepolarizationisinconsequential.However,iftheprematuredepolarizationsoriginateearlyintherelativerefractoryperiodoftheventricles,conductionoftheprematureimpulsefromthesiteoforiginisslow,andhenceacardiacimpulseismorelikelytoreexcitesomemyocardialregionthroughwhichithadpassedpreviously(aphenomenonknownasreentry).Ifthatreentryisirregular(i.e.,ifventricularfibrillationensues),theheartcannotpumpeffectively,anddeathmayresult.
Action potentials evoked early in the relative refractory period are small, and the upstrokes are not very steep (Fig.
16.15 ). The amplitudes and upstroke slopes progressively improve as action potentials are elicited later in the relative refractory period. Recovery of full excitability is much slower than recovery of the fast response. Impulses that arrive early in the relative refractory period are conducted much more slowly than those that arrive late in that period. The long refractory periods also lead to conduction blocks. Even when slow responses recur at low frequency, the fiber may be able to conduct only a fraction of these impulses; for example, in certain conditions, only alternate impulses may be propagated.
•Fig. 16.16 Effectofchangesincyclelength(CL)ontheactionpotentialduration(APD)ofPurkinjefibers.(ModifiedfromSingerD,TenEickRE.Am J Cardiol. 1971;28:381.)
Effects of Cycle Length
Cycle length is the time between successive action potentials. Changes in cycle length alter the duration of the action potential in cardiac cells (
Fig. 16.16
Fig. 16.5 ) and thus change their refractory periods. As a consequence, changes in cycle length are often important factors in the initiation or termination of certain arrhythmias (irregular heart rhythms).
The changes in action potential duration produced by stepwise reductions in cycle length from 2000 to 200 msec in a Purkinje fiber are shown in